Add Page.RenderShortcodes

A layouts/shortcodes/include.html shortcode may look like this:

```html
{{ $p := site.GetPage (.Get 0) }}
{{ $p.RenderShortcodes }}
```

Fixes #7297

resources/page/page.go

@@ -277,6 +277,7 @@ type PageRenderProvider interface {
 // PageWithoutContent is the Page without any of the content methods.
 type PageWithoutContent interface {
 	RawContentProvider
+	RenderShortcodesProvider
 	resource.Resource
 	PageMetaProvider
 	resource.LanguageProvider
@@ -362,6 +363,11 @@ type RawContentProvider interface {
 	RawContent() string
 }
 
+type RenderShortcodesProvider interface {
+	// RenderShortcodes returns RawContent with any shortcodes rendered.
+	RenderShortcodes(context.Context) (template.HTML, error)
+}
+
 // RefProvider provides the methods needed to create reflinks to pages.
 type RefProvider interface {
 	// Ref returns an absolute URl to a page.